The present invention relates to heat transfer element and, more specifically, to a heat transfer element assembly comprised of a stacked array of spaced absorbent plates for use in a rotary regenerative heat exchanger wherein the heat transfer element is heated by contact with the hot gaseous heat exchange fluid and thereafter brought in contact with a cool gaseous heat exchange fluid to which the heat transfer element gives up its heat.
One type of heat exchange apparatus to which the present invention has particular application is the well-known rotary regenerative heater. A typical rotary regenerative heater has a cylindrical rotor divided into compartments in which are disposed and supported assemblies of spaced heat transfer plates which as the rotor turns are alternately exposed to a stream of heating gas and then upon rotation of the rotor to a stream of cooler air or other gaseous fluid to be heated. As the heat transfer plates are exposed to the heating gas, they absorb heat therefrom and then, when exposed to the cool air or other gaseous fluid to be heated, the heat absorbed from the heating gas by the heat transfer plates is transferred to the cooler gas. Most heat exchangers of this type have their heat transfer plates closely stacked in spaced relationship to provide a plurality of passageways between adjacent plates for flowing the heat exchange fluid therebetween.
In such a heat exchanger, the heat transfer capability of a heat exchanger of a given size is a function of the rate of heat transfer between the heat exchange fluid and the plate structure. However, for commercial devices, the utility of a device is determined not alone by the coefficient of heat transfer obtained, but also by other factors such as the resistance to flow of the heat exchange fluid through the device, i.e., the pressure drop, the ease of cleaning the flow passages, the structural integrity of the heat transfer plates, as well as factors such as cost and weight of the plate structure. Ideally, the heat transfer plates will induce a highly turbulent flow through the passages therebetween in order to increase heat transfer from the heat exchange fluid to the plates while at the same time providing relatively low resistance to flow between the passages and also presenting a surface configuration which is readily cleanable.
To clean the heat transfer plates, it has been customary to provide soot blowers which deliver a blast of high pressure air or steam through the passages between the stacked heat transfer plates to dislodge any particulate deposits from the surface thereof and carry them away leaving a relatively clean surface. One problem encountered with this method of cleaning is that the force of the high pressure blowing medium on the relatively thin heat transfer plates can lead to cracking of the plates unless a certain amount of structural rigidity is designed into the stack assembly of heat transfer plates.
One solution of this problem is to crimp the individual heat transfer plates at frequent intervals to provide a series of folds or notches which extend substantially parallel to the flow of air or gas thereover and protrude outwardly away from the plate for a predetermined distance. Then when the plates are stacked together to form the heat transfer element, these folds serve not only to maintain adjacent plates at their proper distance from each other, but also to provide support between adjacent plates so that forces placed on the plates during the soot blowing operation can be equilibrated between the various plates making up the heat transfer element assembly. Heat transfer element configurations incorporating such spacer folds or notches which extend substantially parallel to the flow of air or gas through the element assembly, including single-lobed and bi-lobed notches, are disclosed for example in U.S. Pat. Nos. 1,823,481; 2,023,965: 2,438,851; 2,596,642; 2,696,976: 2,983,486; 3,463,222: 4,396,058: and 4,512,389.
However, in a heat transfer element assembly of the type wherein the heat transfer elements incorporate folds or notches which extend substantially parallel to the flow of air or gas through the element assembly, there exists the potential for nesting or partial nesting of adjacent plates. That is, the folds may become completely or partially superimposed on one another so that the spacing between adjacent plates is lost or substantially reduced thereby degrading heat transfer performance. This may occur from improper installation or movement of the plates relative to each other during normal operation or during the soot blowing procedure.
Such nesting may be precluded by having bi-lobed spacing notches or folds formed in the heat transfer element sheets extend obliquely relative to the general direction of the fluid flow through the assembled element. As disclosed in U.S. Pat. Nos. 3,183,963 and 4,449,573, element plates having such obliquely extending folds or notches are arranged in a stacked array with the folds or notches of adjacent plates crossing each other. Although such configurations do preclude nesting, they may be unnecessarily complicated.